This invention relates to devices and methods for the repair of defects that occur in articular cartilage on the surface of bones, particularly the knee.
Articular cartilage, found at the ends of articulating bone in the body, is typically composed of hyaline cartilage, which has many unique properties that allow it to function effectively as a smooth and lubricious load-bearing surface. However, when injured, hyaline cartilage cells are not typically replaced by new hyaline cartilage cells. Healing is dependent upon the occurrence of bleeding from the underlying bone and formation of scar or reparative cartilage called fibrocartilage. While similar, fibrocartilage does not possess the same unique aspects of native hyaline cartilage and tends to be far less durable.
Hyaline cartilage problems, particularly in knee and hip joints, are generally caused by disease such as occurs with rheumatoid arthritis or wear and tear (osteoarthritis), or secondary to an injury, either acute (sudden), or recurrent and chronic (ongoing). Such cartilage disease or deterioration can compromise the articular surface causing pain and further deterioration of joint function. As a result, various methods have been developed to treat and repair damaged or destroyed articular cartilage.
For smaller defects, traditional options for this type of problem include non-operative therapies (e.g., oral medication or medication by injection into the joint), or performing a surgical procedure called abrasion arthroplasty or abrasion chondralplasty. The principle behind this procedure is to attempt to stimulate natural healing. At the defect site, the bone surface is abraded, removing approximately 1 mm. or less using a high-speed rotary burr or shaving device. This creates an exposed subchondral bone bed that will bleed and will initiate a fibrocartilage healing response. Although this procedure has been widely used over the past two decades and can provide good short term results, (1-3 years), the resulting fibrocartilage surface is seldom able to support long-term weight bearing, particularly in high-activity patients, and is prone to wear.
Another procedure, referred to as the xe2x80x9cmicrofracturexe2x80x9d technique, incorporates similar concepts of creating exposed subchondral bone. During the procedure, the cartilage layer of the chondral defect is removed. Several pathways or xe2x80x9cmicrofracturesxe2x80x9d are created to the subchondral bleeding bone bed by impacting a metal pick or surgical awl at a minimum number of locations within the lesion. By establishing bleeding in the lesion and by creating a pathway to the subchondral bone, a fibrocartilage healing response is initiated, forming a replacement surface. Results for this technique are generally similar to abrasion chondralplasty.
Another known option to treat damaged articular cartilage is a cartilage transplant, referred to as a Mosaicplasty or osteoarticular transfer system (OATS) technique. This involves using a series of dowel cutting instruments to harvest a plug of articular cartilage and subchondral bone from a donor site, which can then be implanted into a core made into the defect site. By repeating this process, transferring a series of plugs, and by placing them in close proximity to one another, in mosaic-like fashion, a new grafted hyaline cartilage surface can be established. The result is a hyaline-like surface interposed with a fibrocartilage healing response between each graft.
This procedure is technically difficult, as all grafts must be taken with the axis of the harvesting coring drill being kept perpendicular to the articular surface at the point of harvest. Also, all graft placement sites must be drilled with the axis of a similar coring tool being kept perpendicular to the articular surface at the point of implantation. Further, all grafts must be placed so that the articular surface portion of these cartilage and bone plugs is delivered to the implantation site and seated at the same level as the surrounding articular surface. If these plugs are not properly placed in relation to the surrounding articular surface, the procedure can have a very detrimental effect on the mating articular surface. If the plugs are placed too far below the level of the surrounding articular surface, no benefit from the procedure will be gained. Further, based on the requirement of perpendicularity on all harvesting and placement sites, the procedure requires many access and approach angles that typically require an open field surgical procedure. Finally, this procedure requires a lengthy post-operative non-weight bearing course.
Transplantation of previously harvested hyaline cartilage cells from the same patient has been utilized in recent years. After the cartilage is removed or harvested, it is cultured in the lab to obtain an increase in the number of cells. These cells are later injected back into the focal defect site and retained by sewing a patch of periosteal tissue over the top of the defect to contain the cells while they heal and mature. The disadvantages of this procedure are its enormous expense, technical complexity, and the need for an open knee surgery. Further, this technique is still considered somewhat experimental and long-term results are unknown. Some early studies have concluded that this approach offers no significant improvement in outcomes over traditional abrasion and microfracture techniques.
U.S. Pat. No. 5,782,835 to Hart et al. discloses an apparatus and method for repair of articular cartilage including a bone plug removal tool, and a bone plug emplacement tool. The method of repairing defective articular cartilage includes the steps of removing the defective cartilage and forming a hole of sufficient depth at the site. A bone plug comprising intact bone and cartilage adhering thereto is removed from a bone lacking defective cartilage is placed in the hole at the site of the damage.
U.S. Pat. No. 5,413,608 to Keller discloses a knee joint endoprosthesis for replacing the articular surfaces of the tibia comprising a bearing part which is anchored on the bone having an upper bearing surface and a rotatable plateau secured on the bearing surface and forming a part of the articular surface to be replaced. A journal rises from the bearing surface and cooperates with a bore in the plateau to provide lateral support.
U.S. Pat. No. 5,632,745 to Schwartz describes a method of surgically implanting into a site a bio-absorbable cartilage repair assembly. The assembly includes a bio-absorbable polygonal T-shaped delivery unit having radial ribs to be mounted in the removed area and a porous bio-absorbable insert supported by and in the delivery unit. The method comprises the steps of preparing the site to receive the assembly by removing a portion of the damaged cartilage and preparing the site to receive the assembly by drilling and countersinking the bone. The assembly is inserted and seated using an impactor in the drilled and countersunk hole in the bone until the assembly is flush with the surrounding articular surface.
U.S. Pat. No. 5,683,466 to Vitale illustrates an articular joint surface replacement system having two opposing components. Each component has a tapered head piece for covering the end of a bone and for acting as an articular surface, an integrally formed screw stem of sufficient length to extend into the bone and inwardly angled bone grips on the underside of the head piece to allow fixation to the bone by compression fit. The partially spherical convex shaped exterior of the first component complements the partially spherical concave shaped exterior of the second component.
U.S. Pat. No. 5,702,401 to Shaffer discloses an intra-articular measuring device including a hollow handle defining a first passageway and a hollow tube having a second passageway extending from the handle, the hollow tube carrying a projection at its distal end for seating on a fixed site and a probe disposed at the distal end of the hollow tube which may be directed to a second site, to enable measurement of the distance between the first and second sites.
U.S. Pat. No. 5,771,310 to Vannah describes a method of mapping the three-dimensional topography of the surface of an object by generating digital data points at a plurality of sample points on said surface, each digital data point including a property value and a position value corresponding to a particular point representing the properties of the surface of the object. A 3-D transducer probe (e.g., a digitizer) is moved on or over the surface along a random path, and the sample points are digitized to generate a real-time topography or map on a computer screen of selected properties of the object, including without limitation, surface elevation, indentation stiffness, elevation of sub-surface layers and temperature.
Prosthetics for total knee replacement (TKR), whereby the entire knee joint or a single compartment of the knee joint is replaced can be a common eventuality for the patient with a large focal defect. Although these patients are also managed with anti-inflammatory medications, eventual erosion of the remaining articular cartilage results in effusion, pain, and loss of mobility and/or activity for the patient. Problems encountered after implanting such prostheses are usually caused by the eventual loosening of the prosthetic due to osteolysis, wear, or deterioration of the cements used to attach the device to the host bones. Further, some prostheses used are actually much larger than the degenerated tissue that needs to be replaced, so that extensive portions of healthy bone are typically removed to accommodate the prostheses. Patients who undergo TKR often face a long and difficult rehabilitation period, and the life span of the TKR is accepted to be approximately 20 years. Accordingly, efforts are made to forgo the TKR procedure for as long as possible.
Accordingly, there is a need for an improved joint surface replacement system that would be effective in restoring a smooth and continuous articulating surface and that would also be as durable as the former hyaline cartilage surface, within the context of a minimally invasive procedure that allows for a nearly immediate return to activity, restoration of lifestyle, and pain relief.
The present invention provides tools and methods for mapping and measuring the articular surface of a joint (or of any bony surface) and for fabricating a prosthetic device based on this recorded data.
In one method consistent with the invention, once the defect of the chondral surface has been identified, a guide pin is inserted arthroscopically. A fixation screw having a tapered distal tip and an aggressive distal end thread form is then driven into the subchondral bone in relation to a reference axis that is approximately central to the defect. The fixation device also serves to define a tangent point to the surrounding articular surface. The screw is driven by a socket type driver that engages a hex-shaped proximal extension. A further cylindrical proximal extension of the screw (or other mating feature, e.g., a recess in the screw) that eventually serves as a fixation element for the surface prosthetic is at this time concealed with a cover (or other mating feature corresponding to the mating feature of the screw, e.g., a plug for mating with a screw having a recess as its mating feature) having a radiused proximal end. One or more milled slots run the length of the uniform diameter portion of the screw.
Under arthroscopic view, the screw depth is adjusted so that the radiused cover surface is positioned tangent to the radius that defines the existing articular surface. At this time, the guide pin is removed and the knee is articulated. The depth positioning of the radiused cover establishes an origin or reference point for all future measuring, cutting, and prosthetic machining operations. Arthroscopic examination is carried out to confirm positioning.
A measuring tool is inserted on the reference axis. A central element of the measuring tool is a static post that establishes the axial location of origin. By rotating the outer arm or outrigger of the measuring tool relative to the static post while also maintaining contact with the articular surface, an axial displacement or Z dimension can be established relative to the origin for any point along the known radial sweep of the outrigger to determine the final geometry of the prosthetic surface which fits within the defect. These Z dimensions can be recorded in real time with conventional dial gauge indicators, or with digital recording devices, or by using marking techniques. Although numerous points may be taken, ideally a minimum number of points are taken to accurately define the target articular surface.
Locating surfaces or features created on the screw, (or alternatively, on the radius cover, as described in alternative embodiments herein), correlate to some surface or feature on the measuring tool and allow the measurement of the rotational position of the points about the axis with respect to the locating surfaces. Data recorded during the mapping procedure can then be entered into parametric engineering design software or similar algorithm to define a three dimensional surface matched to the bearing surface geometry to be implanted and reproduce the anatomic contours mapped.
An alternative measuring device for obtaining the articular surface dimension includes an outer marking element and an inner recording element. The marking element includes a sharp indenting mechanism which when pressed by the surgeon creates a depression or mark in the relatively soft surface of the recording element, which deforms at these marked points so that they can be utilized as patient data. The recording element also includes a surface that corresponds to the surface of the proximal extension of the fixation screw. During the mapping procedure, data points are established of the rotational position of the mapped articular surface relative to the screw. These data points are translated to the implant geometry so that the accurate rotational location of the implant relative to the screw is maintained.
In order to secure the implant to the fixation screw, a precision taper (or other component of a mating feature) is machined into a protrusion (or other component of a mating feature) on the back of the device. The implant may be constructed of cobalt chromium, or other materials. The implant may also include a slight outward taper or protrusion along the diametrical surface to enhance load bearing or load transfer properties of the implant to surrounding bone. Additionally, a series of radial cuts may create surfaces that increase resistance of the implant to rotational forces. These features may be located around the outer diameter of the implant.
In another aspect, the invention includes a compass instrument for measurement and surface preparation of the implant target site subsequent sizing of the implant. This compass instrument is configured so that it can be delivered to the site arthroscopically, and when coupled to the axis defined by the guide pin it can be used for measuring and cutting operations.
In another embodiment, the compass instrument consists of a handle, a cannulated shaft that extends through the handle, and a cannulated distal offset arm configured to serve as a linearly adjustable mounting tool for a series of cutting blades, boring blades, or measuring probes.
With the guide pin advanced through the instrument shaft, when fitted with a blade, a fixed length from the rotational or reference axis to the cutting blade""s cutting surface is established. This defines the radius that is effected as the instrument is rotated around the guide pin, and corresponds to the overall diameter of the implant. This sharp cutting blade is used to circumscribe and cleanly cut the surrounding articular cartilage.
In another aspect, the invention features a bone cutting or scoring instrument whereby the bone-cutting instrument is positioned on the guide pin reference axis and is used to prepare the target site to match in configuration and dimension the contacting surface of the implant. The matching fit between the bone surfaces of the prepared target site and the bone contacting surfaces of the implant can advantageously ensure long term clinical results with the implant, as poor quality of fit between bone surfaces and bone contacting surfaces of traditional orthopedic prosthetic devices has been noted to contribute to early clinical failures.
Following fabrication of the implant, a second surgical procedure is performed. The radiused cover is removed exposing a precision taper (or, alternatively, the cover may be removed during the first procedure). A pin with a distally mounted element is placed through the central lumen of the fixation screw so that the distally mounted element is secured into the screw. This element carries one or more suture strands that now trail from the fixation screw. The sutures are then threaded through the implant and a knot or bead may be created proximal to the implant. By continuing to manipulate and tension the suture strands, the implant can be brought coaxial to the fixation screw. Once coaxial, the implant is aligned via engagement of the keyed elements and driven into place with a plastic driving rod and mallet. Finally, through the guide aperture on the surface of the implant, bone cement may be injected to enhance the contact surface between the implant and the subchondral bone.
In another aspect, the invention further features a driver whereby the implant is connected to the driver via a holder and a tether element, such as a suture or wire. The implant and the driver are then inserted arthroscopically. Tension is then applied to the tether element so that the implant is drawn back and seated on the driver. The implant can then be controllably delivered to the prepared target site. The seat portion of the driver may comprise a material that may be impacted to seat the implant without damaging the implant surface.